In this paper, we propose a new envelope detection (ED) method to reduce the analog-to-digital converters’ (ADCs) sampling rate from 20 kHz to 20 Hz in a double-sideband low-intermediate-frequency (IF) continuous-wave Doppler radar. Compared with the conventional synchronous demodulation (SD) method, which typically consumes a large amount of ADC resources, the proposed ED can achieve the same results on both mechanical vibration and human vital sign detection. Meanwhile, this paper also describes in detail the hardware implementation of carrier compensation and envelope detectors to recover and extract the envelopes in the IF domain. Experiments also show that, when the IF carrier frequency is higher than 1 kHz, the signal-to-noise ratio performance of ED would be comparable to that of SD.